The technology described in this BACKGROUND OF THE INVENTION is well described in the patent and other literature from which the person skilled in the art can determine operating conditions and parameters for carrying out such processes, including temperatures, sulfur conversion and recovery conditions and times, regeneration conditions and times, cooling conditions and times, and the like.
In a Claus process sulfur recovery plant, an acid gas feedstream comprising hydrogen sulfide is partially combusted with oxygen in a furnace (Claus furnace or Claus thermal conversion zone) to produce at least sulfur dioxide which can then react with hydrogen sulfide by the Claus reaction (see Equation (1) hereinbelow) at high temperatures without catalyst being present to produce elemental sulfur (Claus thermal conversion) and/or which can react in the presence of a Claus catalyst with hydrogen sulfide by the Claus reaction to produce elemental sulfur at lower temperatures (Claus catalytic conversion).
Claus catalytic conversion can be effected under conditions including temperature for continuously removing the resulting elemental sulfur in the form of sulfur vapor from the catalytic reaction zone (above the sulfur dewpoint Claus catalytic conversion) or under conditions including temperature (adsorption conditions) for forming and depositing a preponderance of the resulting elemental sulfur on the Claus catalyst (adsorptive Claus catalytic conversion). In the former process, sulfur can be removed continuously from the resulting gas-in-process by a sulfur condenser. In the latter process, the catalyst having sulfur deposited thereon can be periodically heated to vaporize sulfur from the catalyst, followed by removing sulfur from the resulting gas-in-process by a sulfur condenser. This periodic heating can regenerate the catalyst and restores a high level of activity and can be followed by a cooling period to return the catalyst to a lower temperature for operation under adsorption conditions. Frequently also, Claus catalytic conversion can be effected in a series of Claus catalytic conversion zones with one or more above the dewpoint Claus catalytic conversion zones followed by one or more adsorptive Claus catalytic conversion zones, since lower temperatures in successive reactors favors removal of sulfur compounds from the gas-in-process to lower levels.
Regeneration can be effected by streams richer or leaner in sulfur and sulfur compounds (elemental sulfur, hydrogen sulfide, and sulfur dioxide) than will contact the catalyst when operating under adsorption conditions. It can be desirable, in fact, to regenerate catalyst with streams richer in hydrogen sulfide and sulfur dioxide, because this allows regeneration to be effected concurrently with above the dewpoint Claus catalytic conversion, thereby reducing downtime for a Claus catalytic reactor and potentially reducing the number of Claus catalytic reactors required for achieving a given level of overall sulfur recovery.
A significant problem in maintaining Claus catalyst activity in such plants as hereinabove described is that of preventing or reversing catalyst sulfation. The build-up of sulfate on catalyst surface diminishes catalyst activity and reduces recovery in a plant. It is known that sulfation can occur in regard to low temperature Claus adsorption both during adsorption function or during regeneration function.
Sulfation of Claus catalyst is an equilibrium phenomenon dependent on the concentration, especially of O2, SO2, and SO3 in the Claus process gases, and the temperatures utilized with the catalyst. Sulfation can be reduced from the catalyst by hydrogen sulfide to form elemental sulfur and water vapor. Thus, it is known to reverse catalyst sulfation by passing Claus plant feedstreams comprising essentially only hydrogen sulfide as a sulfur species in contact with sulfated catalyst at a temperature at least higher than that at which the sulfation occurred. If the catalyst temperature in the presence of the hydrogen sulfide is greater than the temperature at which the sulfate was formed, then sulfate levels are reduced below previous equilibrium levels. The rate and degree of sulfate reduction by hydrogen sulfide thus depends upon temperature, hydrogen sulfide partial pressure, and the original temperature of the sulfate formation.
It is well known that the temperature required for reversal of catalyst sulfation must exceed to some extent the temperature of sulfate formation for significant reversal of catalyst sulfation to occur. Thus, it has been recommended to reactivate catalyst which has lost activity using hydrogen sulfide free of sulfur dioxide at temperatures of 190.degree.-350.degree. C. (375.degree.-660.degree. F.) where adsorption of elemental sulfur on catalyst occurs in the range of 110.degree. C. to 160.degree. C. (230.degree.-320.degree. F.) (see British Patent No. 1,307,716). Generally, it is recommended to reverse sulfation by using a hydrogen sulfide stream having an inlet temperature at least 50.degree. F. (28.degree. C.) above the temperature at which sulfation occurs, although it is noted that any increase in the temperature employed during treatment with a hydrogen sulfide-containing stream, even if only 20.degree. F. (11.degree. C.) is of some benefit in reducing sulfation. See, for example, W. S. Norman, "There are Ways to Smoother Operation of Claus Plants," The Oil and Gas Journal, Nov. 15, 1976, pp. 55-60.